There is a multitude of filtration devices which separate a feed stock into filtrate and retained suspended matter which is too large to pass through the pore structure of the filter. A straight-through filter retains the suspended matter on the filter surface or within the filter matrix and passes only the filtrate. Cross-flow filters operate with tangential flow across the filter surface to sweep away suspended matter unable to pass through the filter surface pores. Cross-flow filters provide for the continuous extraction of retentate, or concentrated suspended matter, from one portion of the device and continuous extraction of filtrate from another portion.
As is well-known in the art, the filtration rate of cross-flow filters is generally limited by the resistance of a filter cake that builds up on the filter surface. The thickness and corresponding resistance of this cake is controlled by the cross-flow velocity. This phenomenon of cake thickness controlled by concentration polarization of retained suspended matter is extensively described in the technical literature. In order to obtain the maximum filtration rate, cross-flow filters are normally constructed from porous materials which have a low resistance to filtrate flow relative to that of the filter cake. That is, in operation the pressure drop across the porous filter itself is low relative to the pressure drop across the filter cake, and the resistance of the latter is determined by hydrodynamic flow conditions across the filter surface.
Cross-flow filters can be constructed using multiple-passageway, porous monoliths. Such monoliths can have tens to thousands of passageways extending through them, with the passageways normally parallel and uniformly spaced. When in use the feed stock is introduced under pressure at one end of the monolith, flows in parallel through the passageways, and is withdrawn as retentate at the downstream end of the device.
Filtrate which passes into the porous monolith walls separating the passageways combines and flows through the walls toward the periphery of the monolith, and is removed through the outer skin of the monolith. The resistance to flow in the torturous flow path of the monolith passageway walls can severely limit filtration capacity, and for this reason cross-flow filters based on large diameter, high surface area, multiple passageway, porous monoliths are not found in commercial use unless they incorporate some means to overcome this limitation.
Membrane devices utilize a semipermeable membrane to separate filtrate, also called permeate, from retentate. There is a multitude of different pressure driven membrane devices which separate and concentrate particles, colloids, macromolecules, low molecular weight molecules, and separate gases. Membranes generally require a mechanical support which can be integral with the membrane, or separate. For example, membranes can be coated onto, or simply mechanically supported by, a porous support material.
Multiple-passageway, porous monoliths can be especially useful as membrane supports. In this instance membranes are applied to the passageway walls, which serve both as a mechanical support and as the flow path for filtrate removal to a filtrate collection zone. A high flow resistance of the passageway walls of the monolith can be troublesome first in that it can prevent adequate formation of membranes, for example, by dynamic formation procedures. Second, if membranes are otherwise applied to the monolith passageway walls, the resistance of the passageway walls to filtrate flow can limit device capacity. This limitation has clearly been recognized by developers of such devices, for example, by Hoover and Roberts in U.S. Pat. No. 4,069,157, issued Jan. 17, 1978. That patent teaches a solution to such limitation by limiting a number of parameters to values within specific ranges. The surface area of the passageways per unit volume, the porosity of the support, and the proportion of the volume of the support materials exclusive of the passageways to the total volume of the support are defined within certain ranges, and are combined to define an allowable range of a permeability factor for the support.
Other monolith based membrane devices have been developed in many countries, including the United States, France, Germany, the United Kingdom, the Netherlands, Japan, and The People's Republic of China. For these devices practitioners also have recognized a support permeability limitation and have generally overcome this limitation by use of monoliths with a combination of small overall diameter, relatively few feed passageways and large pore size of the support material. Several commercially available membrane devices utilize a number of small diameter monoliths, each with up to 37 passageways, distributed within a cylindrical housing. Filtrate exits from the sides of each monolith and mixes with the filtrate from the other monoliths, after which it is collected. The overall packing density, or membrane area per unit volume, of these devices is relatively low.
The monoliths used by all the above sources as supports for membrane devices have had the common characteristic of employing passageways which are substantially uniformly spaced throughout the support. Given this constraint, product developers have worked with variables such as those detailed by Hoover and Roberts in the above referenced patent to avoid filtrate flow path limitations.
Thus the flow resistance of the passageway walls of porous monoliths can be a limiting factor in the use of monoliths either as cross-flow filtration devices or as membrane supports in membrane devices. Further this limitation becomes increasingly severe as the packing density, or effective filter or membrane area per unit volume, of the device increases.
Other developers of monolith based devices have used means of filtrate removal other than along the sides of the monolith device. One category of such filtration devices is a balanced pressure system. In this device one or more of the passageways is used to remove filtrate in a longitudinal manner rather than in the radial manner of Hoover and Roberts cited above. Such devices include those of Ellenburg in U.S. Pat. No. 3,712,473, issued Jan. 23,1973; Hoover and Roberts in U.S. Pat. No. 4,032,454, issued Jun. 28, 1977; and Connelly in U.S. Pat. No. 4,222,874, issued Sep. 16, 1980. For these devices a primary reason for selecting this mode of filtrate removal is to be able to pressurize the monolith exterior surface with feed material in order to keep the monolith under a compressive force, thereby minimizing the potential of monolith mechanical failure. Connelly further teaches the use of radial filtrate ducts to reduce resistance to filtrate radial flow within a large diameter monolith to a central longitudinal filtrate duct. Such radial filtrate ducts pass through the porous monolith material and do not intersect any of the longitudinal passageways. This arrangement of radial filtrate ducts is such that it is physically difficult to utilize monoliths of high packing density, and the devices of Connelly have packing densities of below about 100 square feet of feed passageway area per cubic foot of monolith structure.
Yet other developers of monolith based devices have provided modifications to accomplish multiple flow path bodies. For example, in the heat exchange devices of Kelm, U.S. Pat. Nos. 4,041,592, issued Aug. 16, 1977; and 4,126,178, issued Nov. 21, 1978; and Noll et al., U.S. Pat. No. 4,041,591, issued Aug. 16, 1977; two fluids enter separately into a body, are maintained separately within the body, and exit separately. Thermal exchange occurs between the two fluids but there is no transfer of matter. Kelm and Noll et al. state that a porous body can be used for filtration or osmosis processes, but no further teaching is provided.
Still other developers of monolith based devices have provided modifications to provide multiple flow path bodies. Charpin, U.S. Pat. No. 4,427,424, issued Jan. 24, 1984, discloses such devices fabricated from fine pored gas separation membranes. Schnedecker, U.S. Pat. Nos. 4,338,273, issued Jul. 6, 1982; 4,426,762, issued Jan. 24, 1984; and Schnedecker et al., U.S. Pat. No. 4,518,635, May 21, 1985, describe processes for fabricating such devices useful for heat exchange and ultrafiltration.
The devices of Kelm, Noll, Charpin and Schnedecker when considered for filtration or membrane devices all have the characteristic that the transport of matter is considered to occur primarily from high pressure feed passageways through adjoining walls directly into low pressure filtrate passageways. Accordingly, the devices disclosed have each feed passageway adjacent to a filtrate passageway.
The devices of Faber and Frost, U.S. Pat. No. 5,641,332, issued Jun. 24, 1997, disclose single monolith based membrane devices which have passageway walls of varying thickness as a means of addressing permeate carrying capacity of monoliths when used as membrane supports. Analogously, the devices of Yorita et al., U.S. Pat. No. 5,855,781, issued Jan. 5, 1999, disclose single monolith devices which have thicker and thinner sections of monolith passageway walls, with filtrate conduit holes drilled through the thicker sections. Yorita et al., also disclose single monoliths with complex open slots which form filtrate conduits.
Goldsmith, U.S. Pat. Nos. 4,781,831, issued Nov. 1, 1988; 5,009,781, issued Apr. 23, 1991; and 5,108,601, issued Apr. 28, 1992; herein incorporated by reference, disclose a variety of monolith structures used as cross-flow filtration devices and membrane devices. These structures are based on individual large-diameter monoliths with filtrate conduits formed with the monolith as well as structures of closely packed monoliths with filtrate conduits formed by the spaces among the individual monoliths. These devices allowed different means of filtrate removal, including along the side or sides of a structure or extracted through tubes or ducts and the ends of the structures. These devices include either single large monoliths with internal filtrate conduits or closely packed assemblies of smaller monoliths without internal filtrate conduits, but not both.
Important considerations in the production costs of cross-flow filtration and membrane devices include both the cost of the monolith and the labor costs for handling the monoliths. For small diameter (or other characteristic dimension) monoliths, the cost for equipment to extrude, dry and fire the monoliths is modest. Equipment costs to produce larger diameter monoliths can be quite high. Further, monoliths of larger diameter are difficult to dry and fire and achieve high production yields. These considerations are affected by monolith material and passageway structure, among many variables. But, generally, monoliths of diameter much above seven inches require expensive production equipment and can have very low production yields. Considering labor costs, small dimension monoliths have relatively high labor costs per unit filter or membrane area. As monoliths become larger, labor costs per unit filter area, or unit membrane area, decline. For this reason, costs per unit area for a monolith- based cross-flow filtration or membrane device can be expected to pass through a minimum with increasing monolith diameter.
It is within this background that the present invention is to be considered. More specifically, all existing devices based on use of multiple passageway monoliths as cross-flow filtration or membrane devices use either an assembly of relatively small diameter monoliths or single large diameter monoliths with internal filtrate conduits. No prior art discloses an assembly of monoliths which themselves contain one or more internal filtrate conduits.